1. Field of the Invention
The present invention relates to a process of producing an electrophotographic photosensitive member, and more specifically, it relates to a process of producing an electrophotographic photosensitive member having a photosensitive layer containing two or more kinds of charge generating materials.
2. Related Background Art
In recent years, various kinds of organic photoconductive materials for electrophotography have been developed, and electrophotographic photosensitive members using the organic photoconductive materials are carried in many copying machines and printers.
The organic photoconductive materials have a relatively high degree of freedom of molecular design and also permit spectrosensitive design. However the organic photoconductive materials having sufficient sensitivity to a semiconductor laser beam are not abundant. Here, the above-mentioned semiconductor laser beam has an oscillating wave length of from about 780 to 800 nm which can be used in laser beam printers and laser facsimiles to which much attention is paid these days. Additionally, the spectrosensitivity range of the organic photoconductive materials is limited.
For example, in designing an electrophotographic photosensitive member which is equipped with the composite function of the copying machine using plain paper and the laser beam printer or laser facsimile, sufficient spectrosensitivity is required in an expanded range inclusive of from a visible range of about 400 nm to an infrared range of about 800 nm which is the wave length range of the semiconductor laser beam. However it is difficult to obtain such a spectrosensitivity from a single charge generating material.
In view of the foregoing, it can be conceived to combine a plurality of charge generating materials which are sensitive in different wave length ranges, for example, a material having excellent sensitivity to visible light and another material having excellent sensitivity to long wave length light. However the following points make it very difficult to obtain a suitable mixing condition of the plural materials in a photosensitive layer.
That is, the photosensitive layer can usually be formed by coating an electroconductive support with a coating solution containing the organic photoconductive materials, a binder resin and a solvent. However, when the two or more kinds of charge generating materials are present in one coating solution, the charge generating materials agglomerate owing to the difference between their .zeta. potentials, and as a result, they precipitate. Additionally, in such a case, a suitable solvent for each may be different depending on the nature of the charge generating materials, so that the crystal conversion of the charge generating materials may occur. For these reasons, it is difficult for all the charge generating materials to be stably present in the coating solution.
Furthermore, in case that each of the coating solutions is prepared for each of the charge generating materials and the electroconductive support is coated in turn with the coating solutions in accordance with a dip-coating process, the lower charge generating layer dissolves out, depending upon the kinds of binder resin and solvent to be used. Consequently stable electrophotographic characteristics cannot be obtained.
Moreover, when a curable resin is used in the layer containing the charge generating materials with the intention of removing the above-mentioned drawbacks, the formation of a three-dimensional structure is impeded by the charge generating materials present in the resin. Even if the resin is cured, the resistance of the layer increases, with the result that the electrophotographic characteristics deteriorate. Additionally, in case that a curing agent or the like is contained in the layer, there is still the problem that the electrophotographic characteristics are poor.
An example in which a plurality of charge generating materials are used is disclosed in Japanese Patent Laid-open No. 50-75042. In this example, the charge generating layer may be formed by means of spray coating. However, this publication does not refer to solving a technical problem of the present invention such as the agglomeration of the charge generating materials in the coating solution. In the publication, there are neither a description regarding independent spray devices for the respective charge generating materials nor a detailed description of these spray devices.